Biol Invasions DOI 10.1007/s10530-017-1560-8 ORIGINAL PAPER Habitat fragmentation provides a competitive advantage to an invasive tree squirrel, Sciurus carolinensis Tyler Jessen . Yiwei Wang . Christopher C. Wilmers Received: 13 April 2017 / Accepted: 2 September 2017 Ó Springer International Publishing AG 2017 Abstract Changes in the composition of biological (Sciurus griseus) by non-native eastern gray tree communities can be elicited by competitive exclusion, squirrels (Sciurus carolinensis). We tested this wherein a species is excluded from viable habitat by a hypothesis along a continuum of invasion across three superior competitor. Yet less is known about the role study sites in central California. We found that within of environmental change in facilitating or mitigating the developed areas of the University of California at exclusion in the context of invasive species. In these Santa Cruz campus and city of Santa Cruz, S. situations, decline in a native species can be due to the carolinensis excluded S. griseus from viable habitat. effects of habitat change, or due to direct effects from The competitive advantage of S. carolinensis may be invasive species themselves. This is summarized by due to morphological and/or behavioral adaptation to the ‘‘driver-passenger’’ concept of native species loss. terrestrial life in fragmented hardwood forests. We We present a multi-year study of tree squirrels that classify S. carolinensis as a ‘‘driver’’ of the decline of tested the hypothesis that tree canopy fragmentation, native S. griseus in areas with high tree canopy often a result of human development, influenced the fragmentation. Future habitat fragmentation in west- replacement of native western gray tree squirrels ern North America may result in similar invasion dynamics between these species. Our study warrants consideration of existing and predicted interactions & T. Jessen ( ) Á C. C. Wilmers between potentially invasive species that co-occur Environmental Studies Department, University of California, 1156 High Street, Santa Cruz, CA 95064, USA with native species where land use change is proposed. e-mail: [email protected] Keywords Habitat fragmentation Á Invasive T. Jessen species Á Competitive exclusion Á Squirrel Á Sciurus Ecology and Evolutionary Biology Department, University of California, 1156 High Street, Santa Cruz, carolinensis Á Sciurus griseus CA 95064, USA Y. Wang San Francisco Bay Bird Observatory, 524 Valley Way, Milpitas, CA 95035, USA Introduction Present Address: Understanding the effects of human development on T. Jessen the structure of biological communities is critical for Ecosystem and Public Health Department, Faculty of Veteri- nary Medicine, University of Calgary, 3280 Hospital Dr. NW, the successful management and conservation of Calgary, AB T2N 4Z6, Canada species worldwide. Habitat fragmentation, a process 123 T. Jessen et al. by which continuous habitat is subdivided into smaller example, Reynolds (1985) showed that red squirrels parcels surrounded by habitat unlike the original, is (Sciurus vulgaris) native to England were displaced in one of the primary threats to biodiversity globally Eastern England following the introduction of S. (Andren 1994; Fahrig 2000). Anthropogenic habitat carolinensis around the turn of the 20th century. In fragmentation, including urban development, is Washington State, USA, competition with introduced known to alter biological assemblages through S. carolinensis is thought to play a key role in the changes in community dynamics (Saunders et al. recent decline of native western gray tree squirrels 1991; Andren 1994; Fahrig 2000). (Sciurus griseus), which have now been classified as A common example of a community response to ‘‘threatened’’ in the state (Linders and Stinson 2007). habitat fragmentation is competitive exclusion via The ecological mechanisms governing the replace- interspecific competition, in which one species ment of native tree squirrels by invasive S. carolinen- excludes another from a shared niche space (Hardin sis are highly variable, but likely include interference 1961). Competitive exclusion by invasive species is and exploitation competition for shared niche factors often cited as a primary reason for native species such as seed caches and nest sites (Wauters et al. decline (Sakai et al. 2001; Vila and Weiner 2004). 2000, 2002b). However, less is known about the role of landscape Sciurus carolinensis was introduced to the San change, including habitat fragmentation, in facilitating Francisco Bay area in the 1970’s and has since spread or mitigating biological invasions (Holway 1999; across central California, including the counties of Mack et al. 2000; Gonzales et al. 2008; Lawes and Santa Clara, San Mateo, and Santa Cruz (Hall 1981; Grice 2010; Bennet et al. 2011). Part of the difficulty in Carraway and Verts 1994). Its exact range is unknown, understanding the cause of native species decline in but it is frequently documented in urban areas such as this context is that landscape change can covary with city parks. For example, Haff et al. (2008) qualita- invasive species impacts (Didham et al. 2005). The tively reported that S. carolinensis is commonly ‘‘driver-passenger’’ model of native species loss in the observed in the core University of California at Santa context of invasive species and habitat modification is Cruz (UCSC) campus grounds whereas the native a theoretical framework that attempts to disentangle western gray squirrels, S. griseus, are rarely seen near these effects (Didham et al. 2005; MacDougall and the center of the campus. The ecological mechanisms Turkington 2005). In this model, an invasive species is underpinning the distributions of non-native S. caro- considered a ‘‘driver’’ of native species decline when linensis and native S. griseus in North America are they compete directly with, and exclude or eliminate poorly understood, but examining the drivers of tree native species. In contrast, an invasive species is squirrel community structure in response to landscape considered a ‘‘passenger’’ when landscape change change will lead to a better understanding of invasion causes a decline in native species, and the invader dynamics and native species loss. subsequently occupies areas previously inhabited by This study tests the hypothesis that tree canopy natives. While experimental tests of this hypothesis fragmentation resulting from human development is have been performed (e.g. MacDougall and Turking- facilitating the competitive exclusion of S. griseus by ton 2005), there is a lack of quantitative field studies to S. carolinensis, and that S. carolinensis is therefore a identify invasive species as ‘‘drivers’’ or ‘‘passengers’’ ‘‘driver’’ of native species loss. We defined habitat of native species loss in modified environments (With fragmentation in this study as the loss of tree canopy 2004; Grarock et al. 2013). cover, since canopy cover is an important feature for The eastern gray squirrel (Sciurus carolinensis)isa tree squirrels and forest loss is a known cause of rodent native to eastern North America that has been habitat fragmentation (Laurance et al. 2002). We also introduced by humans to many areas of the world, test the alternative hypothesis that human develop- including England, Italy, South Africa, and western ment is the direct cause of decline in S. griseus, and North America (Steele and Koprowski 2001). These that S. carolinensis is therefore a ‘‘passenger’’ of introductions have often been followed by a displace- landscape change. Finally, we also test the hypothesis ment or population decline of native tree squirrels that forest composition has an effect on the distribu- (Reynolds 1985; Gurnell 1987; Skelcher 1997; tion and abundance of these species, and that any Bruemmer et al. 2000; Wauters et al. 2002a). For 123 Habitat fragmentation provides a competitive advantage observed differences in population metrics between because S. griseus, but not S. carolinensis, has been these species are the result of habitat preference. observed there (possibly due to the geographic barrier Using a combination of observational and hair of the San Francisco Bay). Each area is considered snare surveys, we index the relative abundance of temperate, with dry, warm summers and mild, wet native and non-native tree squirrels and compare winters. The UCSC campus is dominated by mixed- estimates of canopy cover, development cover, and evergreen coastal forests, redwood forests, and grass- forest type along a spectrum of invasion at three study lands, while the cities of Santa Cruz and Santa Rosa sites: no invasion (Santa Rosa), to partial invasion are urban areas with populations of approximately (UCSC), to full invasion (Santa Cruz). Where these 60,000 and 170,000 people respectively (Thomas species cohabitate (i.e. UCSC and the city of Santa 1961; Haff et al. 2008). All of these areas are known to Cruz), we predict that areas of high development are support populations of tree squirrels because of the characterized by low canopy cover and that these areas abundance of nut and pine bearing trees such as the will be dominated by S. carolinensis. We further coast live oak (Quercus agrifolia), tan oak (Quercus predict that S. griseus can be found in areas with high parvula var. shrevei), and knobcone pine (Pinus development cover but only when S. carolinensis is attenuata) (Haff et al. 2008). These trees are used by not present (i.e. the city of Santa Rosa). Identifying the squirrels for food, movement, and